Sticking Lycoming O-360 valve again?

If you are using auto gas in that engine there is no way of knowing
what may be causing your valve problem. Auto gas is not refined and
adultered to any specific standard, certainly not to meet av gas
requirements. I have worked as an engineer in the petrochemical
industry and can assure you that auto gas may have any number of
additives that would cause valve problems in an av engine.

BTW -

How often do you change oil? Confirm you have a full flow filter (it
reads like you do). Does your oil filter have gray (lead) sludge in
it?

What fraction of the time since you got the engine has been 100LL vs
autofuel? I assume the autofuel has to be 91 octane autofuel to handle
your compression?

Have you ever tried to do a mag check at cruise? (i. e. could one mag
be somehow intermittent at temperature?).

Be sure to check inside the rocker covers to see if the rocker shafts
are wearing on the cover sides. I found one (but only one) in my 172M
(1700 TT over 31 years) that was worn substantially. Later shafts have
a nylon thrust boss.

You gotta post what you find. THX

: How often do you change oil? Confirm you have a full flow filter (it
: reads like you do). Does your oil filter have gray (lead) sludge in
: it?
Full flow filter, 50 hours, Aeroshell 80/100 depending on winter/summer. No
lead sludge in the filter, and very tiny (i.e. normal) amount of metal and carbon in
the filter and its element.

: What fraction of the time since you got the engine has been 100LL vs
: autofuel? I assume the autofuel has to be 91 octane autofuel to handle
: your compression?

93 A.K.I autofuel is used for local cruise. Generally at least 25% 100LL on
the takeoff tank. I'd say in the 3-400 hours since we've run it, probably about 60%
on autofuel.

: Have you ever tried to do a mag check at cruise? (i. e. could one mag
: be somehow intermittent at temperature?).

Done in-cruise mag checks. All fine. I generally check it along with my
leaning to verify no rough on just one mag.

: Be sure to check inside the rocker covers to see if the rocker shafts
: are wearing on the cover sides. I found one (but only one) in my 172M
: (1700 TT over 31 years) that was worn substantially. Later shafts have
: a nylon thrust boss.

Will look when I get them off. Probably take a look on Saturday.

: You gotta post what you find. THX

Will do.
-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

Whatever you do - don't use abrasives of any sort on bronze. If you
do, it may charge the bronze with abrasive material making it into a
lap. Be careful with the reamer too.

wrote:
: Other suggestions? I'm figuring to pull off the rocker-box covers, push on
: the valves, and see if I can feel friction in any of them.

Well, I got around to doing this today with my partner and mechanic. Pulled off the valve
covers and pulled off the rocker arms. Unfortunately, pressing on the valves did not reveal any
obvious sticking. Last time we did this (3 years and about 350 hours ago), you could push on the
valves and feel the ones that were sticking. Now, they all seem free.

After pulling off the exhaust and looking up in there, there was little more joy. Very
minimal deposits on the ports, bottom side of the valve, and stems. When having someone else depress
the valve and "wiggle" it side-to-side, it appeared to move some. It almost seems like the guide may
have trapezoidal holes... larger on the valve head side than on the rocker arm side of the guides.

The current plan is to get a tool to compress the valve springs so I can remove them. Maybe
I'll be able to get a better "feel" to them to see if maybe they're sticking in a certain rotation.
It'll also be necessary if I'm going to do SB388C.

Argh! I was *hoping* to find something obvious... so far, no joy. I don't see how even
with loose valve guides, the power could reduce like that. As far as I understand it, loose valve
guides *lead* to sticking valves.

Another possibility I thought of is if the holes are trapezoidal, being a bit tight on the
rocker-arm side, maybe taking off with CHT below 300 is the cause. I generally will get at least 200
CHT before departing, but try not to get over 300 to keep them from getting too hot on climbout.
Maybe the valves heat up faster than the cylinder heads and guides so they bind a little bit until
the CHT comes up and stabilizes. Seems like a long shot.

Oh, and the exhaust isn't in great shape, but I don't think it could be the problem. The
baffles are warped a bit inside, but still all in one piece.

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************